The present invention relates to a magnetic toner for developing electrostatic latent images in recording methods utilizing electrophotography, electrostatic recording, magnetic recording, toner jetting, etc.
Hitherto, a large number of electrophotographic processes have been known. Generally, in these prcesses, an electrostatic latent image is formed on an electrostatic image-bearing member (hereinafter also called a xe2x80x9cphotosensitive memberxe2x80x9d) utilizing ordinarily a photoconductive material, the latent image is then developed with a toner to form a visible toner image, and the toner image, after being transferred as desired onto a transfer(-receiving) material such as paper, is fixed onto the transfer material by application of heat, pressure, heat and pressure, etc., to provide a product copy or print.
As a method for visualizing the electrostatic latent image with a toner, there have been used the cascade developing method, the magnetic brush developing method, the pressure developing method, the magnetic brush developing method using a two-component developer comprising a carrier and a toner, the non-contact mono-component developing method wherein a toner on a toner-carrying member is caused to jump onto a photosensitive member disposed in no contact with the toner-carrying member; the contact mono-component developing method wherein a toner on a toner-carrying member pressed against a photosensitive member is transferred to the photosensitive member under an electric field, and further the so-called jumping method wherein a magnetic toner carried on a rotatory sleeve (as a toner-carrying member) in which a magnetic role is disposed is caused to jump under an electric field from the sleeve onto the photosensitive member.
As a technical trend of an electrophotographic apparatus, such as a printer, higher resolutions of 1200 dpi and 2400 dpi are desired from a conventional level of 300 dpi or 600 dpi. Accordingly, the developing scheme is required of a higher resolution correspondingly. Also, a copying machine is required to achieve higher functions, so that a digital image forming technique is predominant. This is principally achieved by using a laser beam for forming electrostatic images, and a higher resolution is desired, thus requiring a high-resolution and high-definition developing scheme.
A magnetic developer (hereinafter simply represented as a xe2x80x9cmagnetic tonerxe2x80x9d) used in the jumping method comprises fine particles containing a particulate form of magnetic material such as triiron tetroxide (magnetite) uniformly dispersed in a binder resin together with a wax for improving the fixability. Hitherto, various proposals have been made regarding toner production conditions, surface property and shape of the magnetic material, and species and viscoelasticity of the binder resin, for uniformizing the dispersion state of the magnetic material. However, even a magnetic toner containing a uniformly dispersed magnetic material as described and capable of realizing a satisfactorily high resolution is liable to exhibit insufficient performances in continuous image formation on a large number of sheets in an environment of high temperature/high humidity or low humidity. For example, in the case of continuous formation of high-areal percentage images in a high temperature/high humidity environment, the resolution is liable to be lowered to result in inferior thin line reproducibility, and in the case of continuous formation of low-areal percentage images in a low humidity environment, the resolution may be retained at a satisfactory level, but the density uniformity of a solid image is liable to be impaired. Thus, there is left a room for improvement regarding satisfaction of both the resolutions and the solid image uniformity. The use of a small-particle size and spherical toner has been known as an effective means for improving the image quality, and such a toner is disclosed in JP-A 9-62029 and EP-A 1058157. However, further improvements in environmental stability and image quality are expected.
JP-A 2002-148853 has disclosed an effect of specifying saturated magnetization of the fine powder fraction and the coarse powder fraction for improving the developing performance. However, further improvement in image quality is still desired.
A generic object of the present invention is to provide a magnetic toner having solved the above-mentioned problems of the prior art.
A more specific object of the present invention is to provide a magnetic toner exhibiting a high coloring power, capable of satisfying both thin-line reproducibility and solid image density uniformity without being affected by changes in environmental conditions and areal percentages of images and capable of maintaining high image quality for a long period.
In view of diversity of environments and conditions for use of a toner, we have made an extensive study for stabilization of image qualities even in the case of changes in environments and conditions for toner use, and as a result, it has been found possible to solve the problem by using a toner satisfying a particular specific gravity distribution characteristic whereby the present invention has been arrived at.
More specifically, according to the present invention, there is provided a magnetic toner, having a dry specific gravity of (A) and comprising toner particles each comprising at least a binder resin and iron oxide dispersed therein, wherein the magnetic toner has a specific gravity distribution of toner particle fractions obtainable through wet sedimentation and including:
at most 15 wt. % of a fraction having a specific gravity of above (A)xc3x971.000 and at most (A)xc3x971.025,
0.1-20 wt. % of a fraction having a specific gravity of above (A)xc3x970.975 and at most (A)xc3x971.000,
at least 30 wt. % of a fraction having a specific gravity of above (A)xc3x970.950 and at most (A)xc3x970.975,
0.1-20 wt. % of a fraction having a specific gravity of above (A)xc3x970.925 and at most (A)xc3x970.950, and
at most 15 wt. % of a fraction having a specific gravity of above (A)xc3x970.900 and at most (A)xc3x970.925.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.